Proteins are expressed by genes in bacteria or cells. In the expression process, isoforms of the originally intended target protein can be produced due to glycosylation, glycation, or modifications caused by enzymes or other proteins, and isoforms can also be produced due to deamidation, oxidation, or substitution/deletion of amino acids, which is caused by external environmental stress. In the former case, isoforms are frequently produced by post-translational modifications during cell culture, and in the latter case, isoforms are frequently produced during the purification process or storage.
Isoforms produced as described above are classified into product-related substances and product-related impurities, according to how they maintain their medicinal effects compared to that of the originally intended target protein.
Monoclonal antibodies representative of protein therapeutic agents become a very attractive tool, because all the portions thereof associated with the ability to bind specifically to their targets can be used in vivo.
In the case of monoclonal antibodies, various types of isoforms are produced during expression of the antibodies in cells. In order to maximize the medicinal effects of such isoforms without producing product-related impurities, clone selection and culture processes are required to be developed to achieve this purpose. However, if isoforms corresponding to product-related impurities continue to remain, an effort is required so that these isoforms no longer increase during the purification process or storage.
Because product-related impurities have properties very similar to those of the desired product, it is very difficult to isolate these impurities by chromatography which is used in the purification process. However, with the recent development of technology, attempts have been made to remove such product-related impurities through development of new purification processes.